This invention relates to hydraulic and electro-hydraulic systems for vehicles and, in particular, to an electro-hydraulic system for driving a cooling fan motor and operating a power steering system.
A variety of drive systems are known in the vehicle manufacturing industry for driving a cooling fan used to cool a vehicle engine. For example, hydro-mechanical systems have been developed and provided for large vehicles such as buses. However, these systems consume a high engine horsepower to drive the hydraulic system making the fuel consumption relatively high.
Electro-hydraulic systems to operate a cooling fan in a vehicle and to operate other vehicle systems have also been proposed in the past. An advantage of such a system to operate a cooling fan for a diesel engine is that it can produce a more constant RPM of the fan on demand with variable engine RPM. Compared to known hydro-mechanical systems, the advantages of electro-hydraulic systems include optimized energy usage, space and weight reduction, and ease of adjustment of the electronic controls in the field. With an electro-hydraulic system, it should be possible to adjust the input parameters more readily, thus making such a system more compatible with various engines and the fan efficiency performance curve.
U.S. Pat. No. 4,798,050 issued Jan. 17, 1989 to Toyota Jidosha Kabushiki Kaisha et al. describes a hydraulic system for a vehicle which employs a hydraulic tandem pump, including a primary pump for supplying fluid under pressure to a power steering device and a secondary pump for supplying fluid under pressure to a hydraulic motor of an engine cooling fan. The two pumps are mounted on a common drive shaft for rotation therewith. The control system for the tandem pump includes an electrically operated flow control valve disposed within a communication passage between the secondary pump and the hydraulic motor to bypass fluid under pressure discharged from the secondary pump into an inlet passage connecting a fluid reservoir to the secondary pump. A temperature sensor is arranged to detect an ambient temperature of a prime engine of the vehicle for producing a signal indicative of this temperature. An electrical control apparatus is connected to this sensor to control a current applied to the control valve in response to the output signal from the sensor.
U.S. Pat. No. 5,875,630 issued Mar. 2, 1999 to Sauer Inc. describes a hydraulic drive assembly employing a variable displacement pump connected in a closed loop circuit with a motor that can be used to drive a fan. An auxiliary pump can be operatively connected to the first mentioned pump, the motor and a reservoir for replenishing fluid losses in the closed loop circuit. In a second version of this hydraulic drive assembly, the steering system pump is mounted on the cooling system pump and uses the same input shaft. An indicated advantage of such an integrated package is that it can save space and simplify the drive connections from the engine. One disadvantage of this known hydraulic drive assembly is that it does use a variable displacement pump which generally speaking is more expensive than and heavier than a fixed displacement pump.
It is an object of the present invention to provide an improved electro-hydraulic system for operating a vehicle cooling fan and for providing hydraulic fluid to a power steering system of the vehicle, which system can be manufactured at a reasonable cost and may produce a more constant RPM of the cooling fan on demand with a variable engine RPM.